A valve pause mechanism of a four-stroke internal combustion engine includes a valve pause mechanism. A valve lifter fitted between a valve cam and a valve stem of a poppet valve is always pressed in a direction in which the valve lifter contacts the valve cam with a lifter spring. However, a slide pin is fitted into a slide pin holder fitted in the valve lifter so that a slide pin can slide in a direction perpendicular to the valve stem. A stem working face in contact with the valve stem of the poppet valve and a stem through hole that the valve stem pierces are both adjacently formed in the slide pin and a slide pin driving mechanism. The slide pin driving mechanism selectively makes the stem working face and the stem through hole face the valve stem by moving the slide pin. A side of the slide pin at the back of the stem working face is chamfered across the stem through hole. A plane perpendicular to the central axis of the stem through hole is formed in a chamfered part and its both ends in a direction of the central axis of the slide pin continue to the peripheral surface of the slide pin in a smooth curve. The aforementioned arrangement provides a valve pause mechanism with a durable, relatively light slide pin.